1. Technical Field
The invention relates to a wireless communication system, a wireless communication apparatus and a wireless communication method using spatial multiplexing, and more particularly, to a wireless communication system, a wireless communication apparatus and a wireless communication method, in which a transmitter and a receiver share channel information to perform closed loop type spatial multiplexing transmission.
In particular, the invention relates to a wireless communication systems a wireless communication apparatus and a wireless communication method, which perform beamforming by obtaining a channel matrix on the basis of training series transmitted from a receiver when a transmitter transmits a packet, and more particularly, to a wireless communication system, a wireless communication apparatus and a wireless communication method, which perform beamforming using the training series transmitted from the transmitter to the receiver when the number of antennas of the transmitter which is a beamformer is smaller than that of the receiver which is a beamformee.
2. Background Art
As a system for removing wire in an existing wired communication method, a wireless network is attracting attention. A standard of the wireless network may be the IEEE (The institute of Electrical and Electronics Engineers) 802.11 or the IEEE 802.15.
For example, in the IEEE 802.11a/g, as a standard of a wireless LAN, an orthogonal frequency division multiplexing (OFDM) modulation method which is one of a multi-carrier method is employed. In the OFDM modulation method, since transmission data is distributed to a plurality of carriers having orthogonal frequencies and is transmitted, the band of each carrier becomes narrow, frequency use efficiency is very high, and resistance to frequency-selective fading interference is strong.
In addition, in the IEEE 802.11a/g standard, a modulation method for accomplishing a communication speed of a maximum of 54 Mbps is supported, but a next-generation wireless LAN standard for realizing a new high bit rate is required.
As one of a technology of realizing a high speed of wireless communication, multi-input multi-output (MIMO) communication is attracting attention. This is a communication method in which both a transmitter side and a receiver side respectively include a plurality of antennas to realize spatially multiplexed streams. The transmitter side performs spatial/temporal encoding and multiplexing of plural pieces of transmission data and distributes and transmits the plural pieces of transmission data to N transmission antennas through channels. The receiver side performs spatial/temporal decoding of reception signals received by M reception antennas through the channels to obtain reception data without crosstalk between the streams (for example, see JP-A-2002-44051 (Patent Document 1)). Ideally, spatial streams corresponding to the smaller number (MIN [N, M]) of the transmission and reception antennas are formed.
According to the MIMO communication method, a transmission capacity can increase according to the number of antennas and a communication speed improvement can be realized, without increasing a frequency band. Since the spatial multiplexing is used, frequency use efficiency is high. The MIMO method uses channel characteristics and is different from a simple transmission/reception adaptive array. For example, in the IEEE 802.11n which is the extension standard of the IEEE 802.11a/g, an OFDM_MIMO method using OFDM in primary modulation is employed. Currently, the IEEE 802.11n is being standardized in a task group n(TGn) and a specification established therein is based on a specification established in Enhanced wireless consortium (EWC) formed on October, 2005.
In the MIMO communication, in order to spatially divide a spatially multiplexed reception signal y into the stream signals x, a channel matrix H is acquired by any method and the spatially multiplexed reception signal needs to be spatially divided into a plurality of original streams using the channel matrix H by a predetermined algorithm.
The channel matrix H is obtained by allowing a transmitter/receiver side to transmit/receive existing training series, estimating the channels by a difference between the actually received signal and the existing series and arranging propagation channels of a combination of transmission and reception antennas in a matrix form. When the number of transmission antennas is N and the number of reception antennas is M, the channel matrix is M×N (row×column) matrix. Accordingly, the transmitter side transmits N training series and the receiver side acquires the channel matrix H using the received training series.
A method of spatially dividing a reception signal is largely classified into an open loop type method in which a receiver independently performs spatial division on the basis of the channel matrix H and a closed loop type method in which a transmitter side gives transmission antenna weights on the basis of the channel matrix to perform adequate beamforming toward a receiver to form an ideal spatial orthogonal channel.
As an open loop type MIMO transmission method, there is a zero force (for example, see A.Benjebbour, H.Murata and S.Yoshida, “Performance of iterative successive detection algorithm for space-time transmission”, Proc. IEEE VTC Spring, vol. 2, pp. 1287-1291, Rhodes. Greece, May 2001 (Non-Patent Document 1)) or a minimum mean square error (MMSE) (for example, see A.Benjebbour, H.Murata and S.Yoshida, “Performance comparison of ordered successive receivers for space-time transmission”, Proc. IEEE VTC Fall, vol. 4, pp. 2053-2057, Atlantic City, USA, September 2001 (Non-Patent Document 2)). The open loop type MIMO transmission method is a relative simple algorithm for obtaining reception weight matrix W for spatially dividing the reception signal from the channel matrix H, in which a feedback operation for sharing the channel information between the transmitter and the receiver is omitted and the transmitter and the receiver independently perform spatial multiplexing transmission.
As an ideal one of a closed loop type MIMO transmission method, a singular value decomposition (SVD)-MIMO method using SVD of the channel matrix H is known (for example, see http://radio3.ee.uec.ac.jp/MIMO(IEICE_TS).pdf (as of Oct. 24, 2003) (Non-Patent Document 3)). In the SVD-MIMO transmission, a numerical matrix having channel information corresponding to antenna pairs as elements, that is, a channel information matrix H, is subjected to the singular value decomposition to obtain UDVH. A transmitter side uses V in a transmission antenna weight matrix and transmits a beamformed packet to a receiver and a receiver side typically gives (UD)−1 as a reception antenna weight matrix. Here, D is a diagonal matrix having square roots of singular values λi corresponding to qualities of the spatial streams in diagonal elements (the subscript i indicates an ith spatial stream). The singular values λi are arranged in the diagonal elements of the diagonal matrix D in ascending order and power ratio distribution or modulation method allocation is performed according to communication quality represented by the level of the singular value with respect to the streams such that a plurality of spatial orthogonal multiplexed propagation channels which are logically independent are realized. The receiver side can extract a plurality of original signal series without crosstalk and theoretically accomplish maximum performance.
In the closed loop type MIMO communication system, adequate beamforming is performed when the transmitter transmits the packet, but information on the channel information needs to be fed back from the receiver side for receiving the packet.
For example, in the EWC HT (High Throughput) MAC (Media Access Control) Specification Version V1.24, two kinds of procedures, that is, “implicit feedback” and “explicit feedback”, are defined as the procedure for feeding back the information on the channel matrix between the transmitter and the receiver.
In the “implicit feedback”, the transmitter estimates a backward channel matrix from the receiver to the transmitter using training series transmitted from the receiver, and a forward channel matrix from the transmitter to the receiver is computed to perform beamforming on the assumption that bidirectional channel characteristics between the transmitter and the receiver are reciprocal. Calibration of an RF circuit in a communication system is performed such that the channel characteristics are reciprocal.
In the “explicit feedback”, the receiver estimates a forward channel matrix from the transmitter to the receiver using training series transmitted from the transmitter and returns a packet including the channel matrix as data to the transmitter, and transmitter performs the beamforming using the received channel matrix. Alternatively, the receiver computes a transmission weight matrix for allowing the transmitter to perform the beamforming from the estimated, channel matrix and returns a packet including the transmission weight matrix as the data to the transmitter. In the explicit feedback, since the weight matrix is computed on the basis of the estimated forward channel matrix, it may not be assumed that the channels are reciprocal.
In view of packet transmission, the transmitter is an initiator and the receiver is a receiver. However, in view of beamforming, the initiator for transmitting the packet is a beamformer and the receiver for receiving the beamformed packet is a beamformee. Communication from the beamformer to the beamformee is referred to as “forward” and communication from the beamformee to the beamformer is referred to as “backward”.
For example, when an access point (AP) transmits a data frame to a client terminal (STA) as the beamformer, according to the implicit feedback, the client terminal as the beamformee may only return the training series to the access point for beamforming.
A frame exchange procedure for transmitting the beamforming from the access point to the client terminal by the implicit feedback will be described with reference to FIG. 12.
First, the access point requests the client terminal to transmit the training series. According to the EWC MAC specification, a link adaptation control field (see FIG. 13) in the HT control field (see FIG. 14) of an MAC frame includes a bit called training request (TRQ) and arranging of 1 in this bit corresponds to the transmission request of the training series.
The client terminal returns a sounding packet. The sounding packet includes the training series corresponding, to the number N of transmission antennas of the access point and the number M of reception antennas of the client terminal. The access point can estimate an N×M backward channel matrix when receiving the sounding packet. The access point computes a forward transmission weight matrix for beamforming using the SVD, an Eigen value decomposition (EVD) or the other matrix decomposition and multiplies transmission signal from the antennas by the transmission weight matrix such that the beamformed packet can be sent to the client terminal. By the beamforming, communication can be performed at a high transmission rate even in a place where the packet was hard to be received in the past.
Subsequently, an operation for allowing the beamformer to perform the beamforming using the training series from the beamformee according to the implicit feedback will be described with reference to FIG. 15. In the same drawing, a STA-A having three antennas is the beamformer and a STA-B having two antennas is the beamformee. In the below-described description or equations, a subscript AB indicates forward transmission from, the STA-A to the STA-B and a subscript BA indicates backward transmission from the STA-B to the STA-A. A numerical subscript corresponds to the antenna number of the corresponding terminal. It is assumed that the channels between the STA-A and the STA-B are reciprocal. Accordingly, a backward channel matrix HBA becomes a transposed matrix of a forward channel matrix HAB (HBA=HABt).
The training series transmitted from the antennas of the STA-B are (tBA1, tBA2) and the signals received by the antennas of the STA-A through a channel HBA are (rBA1, rBA2, rBA3), the following equation is obtained.
                              (                                                                      r                                      BA                    ⁢                                                                                  ⁢                    1                                                                                                                        r                                      BA                    ⁢                                                                                  ⁢                    2                                                                                                                        r                                      BA                    ⁢                                                                                  ⁢                    3                                                                                )                =                              H                          BA              ⁢                                                                            ⁡                      (                                                                                t                                          BA                      ⁢                                                                                          ⁢                      1                                                                                                                                        t                                          BA                      ⁢                                                                                          ⁢                      2                                                                                            )                                              Equation        ⁢                                  ⁢        1            
where, the channel matrix HBA is a 3×2 matrix and expressed by the following equation. But, hij is a channel characteristic value of jth antenna of the STA-B to ith antenna of the STA-A.
                              H                      BA            ⁢                                                                =                  (                                                                      h                  11                                                                              h                  12                                                                                                      h                  21                                                                              h                  22                                                                                                      h                  31                                                                              h                  32                                                              )                                    Equation        ⁢                                  ⁢        2            
When the channel matrix HBA is subjected to singular value decomposition, the following equation is obtained. Here, UBA is a matrix having an inherent normalized vector of HBAHBAH, VBA is an inherent normalized vector of HBAHHBA and DBA is a diagonal matrix having a square root of an inherent vector of HBAHBAH or HBAHHBA as the diagonal elements. In addition, UBA and VBA are unitary matrices and complex conjugate transposed matrices thereof become inverse matrices.HBA=UBADBAVBAH Equation 3
The transmission weight matrix necessary for performing beamforming of the frame transmitted from the STA-A to the STA-B is the matrix VAB obtained by performing the singular value decomposition with respect to the forward channel matrix HAB. Here, since the channels between the STA-A and the STA-B are reciprocal and the backward channel matrix HBA becomes the transposed matrix of the forward channel matrix HAB, the singular value decomposition of the channel matrix HAB is computed as follows.HAB=UABDABVABH=VBA*DBAUBAT   Equation 4
When the reciprocity of the channels is used, a desired transmission weight matrix VAB is expressed by the following equation.VAB=(VABH)H=(UBAT)H=((UBAT)T)*=UBA*   Equation 5
That is, it is possible to perform the beamforming using the complex conjugate matrix of UBA obtained by performing the singular value decomposition with respect to the channel matrix estimated on the basis of the training signal from the STA-B.
If the transmission signal of the STA-A is x and a reception signal from the. STA-B is y, the reception signal becomes Y=HABX in a case where the beamforming is not performed (un-steered), but the reception signal y becomes the following equation in a case where the beamforming are performed by the transmission weight matrix VAB (steered)y=HABVABX=(UABDABVABH)·VABX=UABDABX   Equation 6
Accordingly, the STA-B can perform spatial division of the original stream by multiplying the reception signals by DAB−1UABH as a reception weight.
As described above, according to the implicit feedback, since the burden on the beamformee due to the feedback is reduced, it is suitable for a case where the access point (AP) transmits a data frame to the client terminal STA as the beamformer. However, in this case, the terminal which is the beamformer computes the transmission weight matrix for beamforming by performing the singular value decomposition or the other calculation with respect to the channel matrix estimated from the received training series. This calculation has a high load and the processed load increases depending on the number of streams of the training series transmitted from the beamformee.
In an example shown in FIG. 15, since the number N (=3) of antennas of the STA-A is larger than the number M (=2) of antennas of the STA-B, no problem is caused in the processing capability for beamforming. This is because the STA-A is designed to include the processing capability corresponding to the number N of its own streams and the training series of the spatial streams of N or less are divided, an N×M channel matrix is constructed from the divided training series, and the matrix for beamforming is computed based on the N×M channel matrix.
However, in a case of N<M, that is, the number of antennas of the beamformee is larger than that of the beamformer, problems may be caused because the beamformer does not include the processing capability which exceeds the number of its own spatial streams. When the STA-A can process only streams of N which is equal to the number of antennas, M stream trainings may not be divided or the matrix for beamforming may not be obtained from the N×M estimation channel matrix.
As a method for solving such problems without deteriorating the beamforming characteristic, it may be considered that a channel estimation maximum dimension Mmax corresponding to a rated maximum number of antennas is given to the STA-A as the beamformer (for example, if it is based on the IEEE specification, Mmax=4) and the processing capability for computing the transmission weigh matrix for beamforming is given to the obtained N×Mmax estimation channel matrix.
For example, when the number of antennas of the STA-A is N=2 and the rated maximum number of antennas is Mmax=4, the STA-A can compute only a 2×2 matrix in consideration of the communication with the terminal having the same number of antenna, but needs to compute a 2×4 matrix. In this case, since calculation or processing circuit needs to be doubled, miniaturization or low cost of the apparatus is hard to be realized.